The present invention relates to a method and a system for controlling the engine's idle speed during the engine start warm-up and hot idling periods by supplying a controlled auxiliary air flow to the engine.
Cold engines are generally required to have a faster idle speed than warm engines in order to overcome increased viscous and frictional loads encountered during the engine start and warm-up periods. A prior art solution to this problem employed a temperature responsive device such as bi-metallic valve which adjusts the amount of air flow to the engine through a throttle bypass, auxiliary air delivery system to control the idle engine speed as a function of the engine temperature. However, this prior art system must be designed by taking into account the viscosity of particular lubrication oil and if oil of different viscosity is employed the prior art system fails to achieve the desired result. A further disadvantage of this prior art system resides in the fact that if the engine enounters a variation in load such as the compressor operation for air conditioning or the shifting of the torque converter for automatic transmission from the P (parking) or N (neutral) range to the D (drive) range, the idle engine speed tends to reduce considerably from the desired speed.
Closed loop idle air delivery systems have hitherto been developed to meet these problems. One such prior art system employs an electromagnetic valve of the type having a single solenoid coil and a spring-loaded plunger movable between two positions in response to energization of the coil to open and close the passage of an auxiliary air delivery system connected to the primary air delivery system of the engine at a point downstream of the throttle valve which is controlled in response to manual input. Such electromagnetic valves are usually designed so that when the ignition key switch is turned off the valve closes the air delivery passage to ensure normal engine operations against any failure in the solenoid coil circuitry. However, because of the substantially zero clearance during the valve's closed condition, water vapors carried by crankcase ventilation gases or EGR (exhaust gas recirculation) gases tend to condense around the inner wall of the passage near the valve clearance and eventually forms a block of ice thus shutting off the valve opening when the ambient temperature falls below the freezing point. Although this problem could be eliminated by the use of an additional air valve using a bi-metallic spring, this adds to the complexity and cost of the system and reduces the reliability of the system.